The specific aims of this project are: 1) to compare skeletal muscle lipid content (SMLC) in black vs white children by computed tomography (CT) scan of the mid-thigh, and assess the relationship to in vivo insulin sensitivity; 2) to test the hypothesis that free fatty acid (FFA) - induced insulin resistance is associated with larger increases in intramyocellular lipid (IMCL) in black vs white adolescents; 3) to examine if β-cell insulin secretion in prepubertal black children is more sensitive to the stimulatory effect of FFA than in whites; and 4) to test if the β-cell in black obese adolescents is more susceptible to the lipotoxic effect of FFA compared with whites. The methods to be used are: the well- established CT method as well as Magnetic Resonance Spectroscopy (1H-MRS) to assess SMLC and IMCL; intralipid infusion to elevate circulating FFA levels; the hyperinsulinemic-euglycemic clamp with stable isotopes and indirect calorimetry to measure insulin sensitivity and substrate turnover; the hyperglycemic clamp to assess insulin secretion; DEXA and whole body MRI for body composition assessments.